Minimum Depth of Binary Tree

Description

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

Note: A leaf is a node with no children.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 2

Example 2:

Input: root = [2,null,3,null,4,null,5,null,6]
Output: 5

Constraints:

• The number of nodes in the tree is in the range [0, 10^5].
• -1000 <= Node.val <= 1000

Train of thought

• basic case 1: Root node is null, return 0
• basic case 2: Both child nodes of the root node are null, return 1
• recursive relationship:
  • The minimum depth of binary tree = 1 + the minimum depth of the child nodes of root

Solution

public int minDepth(TreeNode root) {
    // base case 1
    if (root == null) {
        return 0;
    }
    // base case 2
    if (root.left == null && root.right == null) {
        return 1;
    }
    // recursive relationship
    if (root.left == null) {
        return 1 + minDepth(root.right);
    }
    if (root.right == null) {
        return 1 + minDepth(root.left);
    }
    return Math.min(minDepth(root.left), minDepth(root.right)) + 1;
}